1. Field of the Invention
The present invention relates to an electrically readable/writable non-volatile semiconductor storage device.
2. Description of the Related Art
In Recent years, a semiconductor integrated circuit needs an embedded-type non-volatile semiconductor storage device with a relatively small size. The non-bolatile semiconductor storage device is required to be embedded on one chip, and continually retain the written information even if the power turned off. Such demands includes redundancy application in mass storage memory such as DRAM or SRAM, storage application for a code including an encryption key, management application in manufacture history, and the like.
Conventionally, a laser fuse has been used as a storage element for a non-volatile semiconductor storage device for these applications. However, when a laser fuse is used, a problem arises that would increase cost for writing because a specially fuse blow device and the associated blow process are required. In addition, since the minimum dimension of a laser fuse is determined by the wavelength of the laser beam in use, it does not keep in step with refinement of other semiconductor elements, causing a problem that the percentage of area occupied by a laser fuse could gradually increase. Further, since a laser is used to perform write operation, a laser fuse needs to be exposed when writing. As a result, if data write is needed after packaging, such a laser fuse may not be available. Therefore, recent years have raised hopes in electrically writable non-volatile storage elements.
As an example of such electrically writable nor-volatile storage elements, an anti-fuse element with a MOS structure is known in the art (see, for example, Japanese Patent Laid-Open No. (HEI) 5-226599). In a data write operation to the element, data is written by applying a high-voltage to both ends of the element to break down an insulating film. On the other hand, in data read operation, such a low voltage is applied to both ends of the anti-fuse element that would not break down the insulating film. Then, detection is made to determine whether the insulating film is broken down according to the amount of current, large or small, that flows into the anti-fuse. In this way, one-bit information is read. From the above, it can be seen that the anti-fuse element is one of the most promising non-volatile storage elements for future use, since it has such a simple data read/write operation that requires only voltage application to the both ends of the element.
However, mainly the following two problems have been observed at those non-volatile semiconductor storage devices using such an anti-fuse element with a MOS structure.
Firstly, the first problem relates to a “write disturb fault”. In writing data to an anti-fuse element with a MOS structure, it is required to handle such a high voltage to break down the anti-fuse element that could also break down any other elements included in the same device. Thus, the anti-fuse element has only a low tolerance for noise caused by other circuits or for a leak current associated with a minute defect in elements included in a memory cell, which infrequently could cause a possible incorrect write operation to an unexpected memory cell. Such an incorrect write operation is referred to as a “write disturb fault”.
Secondly, the second problem is an “insufficient test coverage”. The anti-fuse element with a MOS structure, which is an irreversible non-volatile storage element that retains data by breaking down its internal structure, may not erase data after the write operation of that data. Thus, it is impossible to write data as a test to ensure the read and write operation. This means that the test coverage is insufficient.
Therefore, it is desirable to provide a non-volatile semiconductor storage device with high reliability that overcomes the above-mentioned problems associated with the write disturb fault and insufficient test coverage.